Experiments resulting in different cardiac conditions such as hypertrophy are currently characterized using cross-sectional areas or myocytes dissociated from the main heart structure. These techniques often obscure the true three-dimensional morphology of the myocytes in an intact heart and furthermore, structural information such as the orientation of the cells and the density of the local vasculature are often lost. Recent innovations in imaging have enabled tissues scanning with greater depth than before. Coupling these technologies with a custom microtome will permit the imaging of whole, intact hearts, that are labeled for specific components. I propose in this study to build and use a two-photon histological-slicer unit in order to image a whole heart. In my Preliminary Studies, I describe techniques used for staining hearts that can label the entire heart's capillary network, the myocyte nuclei, and the matrix surrounding the myocytes in an intact heart. After scanning the whole heart, I will use custom image processing programs to isolate individual myocytes in order to calculate morphological details, including the myocyte volume, surface area, and orientation. The specific aims for this study are: 1: To test the hypothesis that two-photon microscopy combined with a precision cutting microtome can be used to image three-dimensional hearts while keeping the native structure intact. Here I propose adapting an existing two-photon system with a collaborator to section and image entire organs, using a new adaptation with fast-scanning two-photon microscopy. 2: To examine the changes in three-dimensional myocyte morphology and structure in mouse hearts bearing genetic mutations for structural molecules. For this aim I propose to test the hypotheses that different mutations known to cause cardiac hypertrophy will exhibit different three-dimensional morphologies, which will be demonstrated using the system developed in Aim 1. [unreadable] [unreadable]